Mounting base pad means for semiconductor devices and method of preparing same

ABSTRACT

A mounting pad means for use in combination with solid-state semiconductive translating devices and including a base pad with flange walls extending from said base pad, and with device mounting pad means being secured to the surface of the base pad and comprising a thin layer of silicone base rubber. The base pad an flange walls are formed of a generally rigid laminate with a core having outer metal foil layers disposed on opposite surfaces thereof, and wherein the core is composed of a thin layer of silicone base rubber.

This is a divisional of application Ser. No. 06/625,140, filed June 27,1984 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates generally to an improved mounting meansfor use in combination with solid-state semiconductive translatingdevices such as transistors, diodes, or the like, and wherein themounting means provides a heat sink or other means of dissipating heat,as well as a mechanical mounting device. The arrangement of theinvention is such that the mounting means which may be fabricated from aformable metal base, may be either a solid metallic pad or a pad formedas alaminate with outer metal foil layers disposed on opposite surfacesof a thermally conductive pliable pad or layer such as a silicone rubbercore. The arrangement facilitates and permits the use of relatively thinlayers of metal foil such as aluminum or copper, which is stiffened orrendered rigid by the forming of the structure.

It has long been recognized that the proper utilization of solid-statesemiconductive translating devices requires a system for dissipating theheat generated in the normal use and operation of these devices.Specifically, as the power requirements of the semiconductivetranslating devices increases, the need for heat dissipationcorrespondingly increases. While metallic layers, that is layers ofaluminum or copper, may be readily employed for heat-dissipatingmounting means, the cost involved in fabricating, working and mountingsuch devices renders it desirable to employ alternate materials ofconstruction for the mounting means. However, as alternate materials areproposed for ease of handling and fabrication, these alternate meanshave normally suffered from an inability to dissipate thermal energy,and thereby impose an unreasonable constraint or limitation upon theoperational characteristics of the solid-state semiconductivetranslating device mounted thereon. For most purposes, however, metalssuch as aluminum or copper are the material of choice for theseapplications, particularly in view of the electrical and thermalproperties of these materials.

SUMMARY OF THE INVENTION

In accordance with the present invention, therefore, a mounting means isprovided for use in combination with solid-state semiconductivetranslating devices which is highly thermally conductive, and whichemploys a formable metallic base pad in the form of a solid metal sheetor a pair of outer metal foil layers disposed on opposite surfaces of athermally conductive adherent corfe layer such as a silicone rubbercore. The structure is rendered generally self-supporting and rigid soas to facilitate handling and working. In the laminated structure, theutilization of the pair of metallic foil layers on opposite sides of thesilicone base rubber core provide a relatively highly thermallyconductive system for heat dissipation.

In the preparation of the base heat sink, various forms may be employedsuch as an extruded or stamped metal part, a machined metal component, ametallic foil, or a laminated metallic-nonmetallic system. The commonelement between each of these forms is, as indicated, that the thermallyconductive material be deformable as a base pad.

The mounting arrangement also includes a thermally conductive insulatorwhich may be integral with the base heat sink element or semiconductorbase. The thermally conductive insulator material, normally siliconerubber, is applied to the material as a coating, composite, or anadhesively applied material such as may be continuously applied througha coating operation onto the surface of the metallic component. Whilethe thermally conductive material is preferably applied directly to theheat sink surface, in ceratin instances and depending upon the design ofthe transistor or other semiconductor assembly, itmay alternatively beapplied directly to the semiconductor device.

When the thermally conductive insulator material is applied to a surfaceas a coating, various operations may be employed such as spray, dip,brush, roller coating, or silk-screen technique. When the insulator ispre-applied to the surface of the metallic heat sink element, theultimate bonding of the semiconductor device to the pre-formed compositemay also be achieved through conventional adhesive bonding operations orvulcanization.

The system of the present invention is highly useful in combination withsemiconductive translating devices which are destined for mounting uponprinted circuit boards. This system further provides a means forsecuring the semiconductive device to the mounting means so that director adhesive assembly operations can be undertaken so that directassembly operations can be undertaken either on a strip-basis or, incertain instances, on a reel-to-reel basis. Also, the mounting means mayinclude use of a conductive adhesive for electrical coupling to thesolid-state semiconductive device, if desired.

The mounting means of the present invention may be convenientlyfabricated in an elongated strip form. In this connection, thefabrication operation may include the initial preparation of anelongated strip of a generally rigid but deformable metal such as ametallic strip or sheet or as a laminate with outer metal foil layers.Score lines may be formed longitudinally along the elongated strip toprovide fold lines for the devices. Also, deep score lines may be formedtransversely at regularly spaced intervals to form a break-off line forthe separation of individual semiconductor assemblies or mounging padsfrom an elongated strip containing a large number of repeating units.

Therefore, it is a primary object of the present invention to provide animproved mounting means for use in combination with solid-statesemiconductive translating devices which comprises a base pad havingflange walls coupled to opposed side edges thereof, and with the basepade and flange walls comprising a generally rigid but deformable basewith a thermally conductive layer being secured or applied to thesurface thereof.

It is yet a further object of the present invention to provide animproved method for the fabrication of mounting means for use incombination with solid-state semiconductive translating device whereinthere is initially prepared an elongated strip of a generally rigid butdeformable and self-supporting metal pad or metallic laminate pad with acore having outer metal foil layers disposed on opposite surfacesthereof. Parallelly disposed score lines are formed along the strip tofacilitate formation of the mounting means, and wherein transverselydisposed score lines are formed along the elongated strip to formbreak-off lines defining or delineating individual assemblies orindividual mounting pad structures.

Other and further objects of the present invention will become apparentto those skilled in the art upon a study of the following specification,appended claims and accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portion of an elongated strip formedfor the preparation of individual mounts for semiconductive devices, andwith FIG. 1 illustrating the strip and semiconductor mounting means insemi-finished form;

FIG. 2 is a view similar to FIG. 1 and illustrating a modified form ofsemiconductor mounting means prepared in accordance with the presentinvention;

FIG. 3 is a perspective view of a mounting means for use in combinationwith solid-state semiconductive translating devices, and prepared from aportion only of the elongated strip illustrated in FIG. 2;

FIG. 4 is a perspective view of an elongated strip from which aplurality of mounting means for solid-state semiconductive translatingdevices may be fabricated;

FIG. 5 is a perspective view of the elongated strip of FIG. 4, and afterthe strip of FIG. 4 had undergone accordion-type folding;

FIG. 6 is a view similar to FIG. 5, and illustrating a modified form ofstrip from that illustrated in FIG. 4, and incorporating the perforationoperations as illustrated in FIG. 2;

FIG. 7 is a perspective view of a single mounting means portion afterhaving been broken-off of the elongated strip illustrated in FIG. 1;

FIG. 8 is a perspective view of the device illustrated in FIG. 7, butfurther illustrating the device after a solid-state semiconductivetranslating device has been secured to the base pad portion thereof; and

FIG. 9 is a perspective view of the mounting means and solid-statesemiconductive device illustrated in FIG. 8, and further showing thedevice with the lead times being bent downwardly to accommodate mountingon a conventional printed circuit board.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the preferred embodiment of the present invention,and with particular attention being directed to FIG. 1 of the drawings,the elongated strip member generally designated 10 includes a repeatingstructure in the form of a plurality of base pad members 11--11. Thestrip 10 is provided with a pair of generally parallelly disposed scorelilnes 15 and 16 along said elongated strip at a point spaced inwardlyfrom the outer edge surfaces 17 and 18 thereof. Furthermore,spaced-apart transversely arranged score lines as at 20 and 21 areformed along the elongated strip 10 to facilitate a breaking-off ofindividual mounting pad means 11--11 such as the individual pad means11--11 encompassed by brackets 22 and 23 for example.

In order to facilitate mounting of a solid-state semiconductivetranslating device on the surface of the laminate structure, devicemounting electrically insulating-thermally conducting pad means such as25--25 may be secured or bonded to the surface of the central base padportion 26 of the elongated strip 10. Base pad central portion 26 isflanked by flange walls 27 and 28, with flange walls 27 and 28 beingcoupled and integral with base pad portion 26. In other words, flangewalls 27 and 28 are extensions of base pad portion 26, and are coupledto the opposed side edges thereof.

As indicated above, the metallic member 11 is preferably highlythermally conductive. Copper and aluminum are, of course, the mostdesirable materials, since each are widely commercially available in awide variety of thickness dimensions. For most purposes, copper andaluminum foil with a thickness of between about 2 and up to about 200mils is useful. In connection with these materials, it will be notedthat in general, metallic foil thicknesses ranging up to about 125 milsmay be advantageously employed.

The elongated strip illustrated in FIG. 2 shows a modified structure.Specifically, the base portion of the modification shown in FIG. 2includes a pair of outer metal foil layers 12 and 13 disposed onopposite surfaces of a thermally conductive core 14 consisting ofsilicone rubber. The metallic layers, preferably metallic foil layers 12and 13, are highly thermally conductive, and copper and aluminum are thematerials of choice. For most pruposes, the copper and/or aluminum foilis formed with a thickness of between about 2 and 5 mils each. Inconnection with these materials, it will be noted that metallic foilthicknesses ranging up to in excess of 5 mils each may be satisfactorilyemployed.

The core material 14 is preferably silicone base rubber, and siliconebase rubbers are, of course, widely commercially available. Suchsilicone base rubbers have adhesive properties, and hence may be readilyemployed as the core material for securing and bonding layers of metalfoil to opposite surfaces thereof. Preferably, the silicone base rubbermaterial forming the core has a thickness of between about 8 mils and 12mils.

The combination of metallic foil layers on opposite surfaces of a coreprovide a generally rigid laminate with desirable mechanical and thermalproperties. The material is sufficiently durable so as to withstandexpsure to the normal events occurring during manufacture and use ofconventional solid-state semiconductive translating devices.

The structure of FIG. 2 includes a plurality of openings or bores formedas at 30--30 and 31--31. Specifically, bores 30--30 and 31--31 areregularly spaced elliptical bores formed along the flange portion of theelongated strip. These bores are formed generally laterally outwardly ofthe parallelly disposed score lines 33 and 34, and with the major axesof each of the bores being generally normal to the longitudinal axis ofthe elongate strip member. In the illustration of FIG. 2, the axis ofelongate strip member generally desingated 35 is, of course, normal tothe major axis of the individual elliptically shaped bores. Also, thetransverse score lines for the break-off zones are formed generallycoincidentally with the major axes of one laterally disposed pair ofelliptical bores, such as the laterally disposed pair of partialelliptical bores shown partially as at 37 and 38. It will beappreciated, of couse, that the configuration of the individual boresformed in the pad material is not critical. Circular, rectangular, orother regularly shaped bores may, of course, be appropriately utilized.The purpose of the bores is to assist in the flow of cooling air aroundand through the device.

Attention is now directed to FIG. 3 of the drawings wherein mounting pad40 is illustrated formed as a fragmentary portion of strip 35 of FIG. 2.In this view, the edge portions 41 and 42 have been folded inwardly tocreate a channel-like member to receive a semiconductor device along thebase pad central portion 43. For effecting a bond between the bondingpad assembly and a suitable substrate, a second thermally conductiveinsulator pad is formed in opposed relationship to the base pad 43.

Attention is now directed to FIG. 4 of the drawings wherein a modifiedform of elongated strip 50 from which individual mounting means may beformed is illustrated. Specifically, the elongated strip means 50 ofFIG. 4 provides the same ingredients or elements as are available fromthat shown in FIGS. 1 and 2, with the exception being the utilization ofan accordion-fold arrangements as illustrated in FIG. 5, wherein themutually adjacent flange portions 51--51 and 52--52 are folded inaccordion-pleat fashion on opposite sides of a center base pad member asat 54. Score lines are formed at the juncture point between individualmembers 51--51 and 52--52 as at 53 and 54 so as to separate the stripinto individually useful mounting pad means.

With attention being directed to FIG. 6, a mounting means havingperforations formed therein similar to those shown in FIG. 2 isillustrated. In this connection, the pad means 60 is formed in a fashionsimilar to that of FIG. 5, with the exception being the formation of theperforations as at 61-61 between what becomes individual pads uponsevering or breaking along the score lines 62 and 63.

Attention is now directed to FIG. 7 wherein a single mounting means 70is shown. Paad 70 includes the flange elements 71 and 72 disposed onopposite sides of the base pad 73. Score lines are formed as at 74 and75 to permit inward and upward folding of flanges 71 and 72 inaccordance with the arcuate arrows as at 76 and 77. In addition,silicone rubber pads are provided as at 78 and 79. Pad 78 facilitatesdirect mounting of the member 70 onto a chassis surface and/or printedcircuit surface, while pad 79 permits direct mounting of the solid-statesemiconductive translating device. One significant advantage derivedfrom pre-assembly of these devices is the virtual elimination of airentrapment, thereby enhancing the thermal conductivity capability of theoverall assembly.

Attention is now directed to FIG. 8 wherein mounting member 70 isequipped with a solid-state semiconductive translating device such asdevice 81. Device 81 is in the form of an encapsulated solid-statesemiconductive translating device, the back of which consists of ametallic mounting bracket or member 82. A bore may be formed in bracketmember 82 as at 83 to accept a screw for mounting onto a chassis member,if desired for the specific application.

Attention is now directed to FIG. 9 of the drawings wherein the assemblyof FIG. 8 is subjected to a further operation wherein the individualtine leads 84, 85 and 86 are bent downwardly at right angles along thelength thereof so as to permit the direct mounting of the device onto aprinted circuit board.

THE FORMABLE METAL BASE

The formable metal base is utilized as the primary thermal conductingmaterial. As has been indicated, the metallic base is preferably copperor aluminum, because of its desirable thermal, electrical, andmechanical properties, and is available and usable as an extruded metalpart, extruded on an elongated basis, a machined metal part, a metalfoil, either solid metal or a laminated metal structure with a thermallyconductive core. In each of these instances, the structure is such thatit can be readily formed with simple hand tools and without need forheavy metal presses, metal bending equipment, or the like. Also, asindicated above, the thickness of individual layers in a laminate shouldbe selected in the area of about 5 mils, with certain compositestructures up to a thickness of about 125 mils being usable. Assuggested hereinabove, the base pad assembly may be formed in eitherstrip-form, or alternatively on a reel-to-reel basis. Such fabricationtehcniques enhance the rate of production of the devices.

THE THERMAL CONDUCTING INSULATOR

As indicated above, this material is preferably fabricated from siliconerubber, and may be applied as a coating, a composite, or as an adhesivepad. The substance of the present invention eliminates the need for aloose insulator element, such as a mica-pad or the like.

The thermally conducting insulator is preferably applied to the surfaceof the metallic foil, as is indicated in the assemblies of FIGS. 1 and2, for example. In certain applications, the thermally conductinginsulator may be appied to the base of the semiconductor deviceundergoing consideration. Also, the appllication technique is selectedand undertaken in such a way that air entrapment is either avoided orsubstantially reduced so as to increase the overall thermal conductivityproperties of the device.

FORMING THE THERMALLY CONDUCTING INSULATOR

The thermally conducting insulator may be applied and/or formed byconventional techniques including spraying, dipping, brushing,screening, roller coating, or transfer coating techniques. Theseapplicating techniques are commonly used with materials including thedesired material of the present invention, silicone rubber.

In these application techniques, the thermally conducting insulator issecured to the base of the transistor (or other semiconductortranslating device) either directly to the base or to the assembly.Adhesive bonding or vulcanization are usable application techniques.

It will be appreciated, therefore, that the mounting means of thepresent invention facilitates and provides effective use in combinationwith solid-state semiconductive translating devices, and wherein thebase pad is formed of a generally rigid, but both durable anddeformable. The structure may be formed as a metallic plate or foilstructure, or as a laminate with a silicone rubber core having outermetal layer foil layers secured to opposite surfaces thereof. Thecombination of the metal foil layers and the silicone rubger coreprovides a finished product having desirable mechanical properties,along with desired thermal properties including a high degree of thermalconductivity.

SURFACE COATING

The thermally conductive insulative material is formed as a stripe onthe overall heat sink structure. The coating, while preferably appliedto the heat sink device, may alternatively be applied to the metallicbase element of the semiconductor device. As such, a strong adherent andcoherent coating is provided in precisely defined and delineated areas,thereby contributing to the overall heat dissipating qualities of thedevice. Also, it will be appreciated that the surface coating may beemployed as a bonding agent for the assembly for bonding or otherwisesecuring the assembly to a suitable substrate, and/or bonding theseimconductor device to the heat dissipating assembly. As indicatedabove, the utilization of such a surface coating, for practicalpurposes, eliminates the formation of any air barrier which impedes orotherwise reduces the thermal performance.

The surface coating may be either bonded through a vulcanizingoperation, or otherwise. The stripe may be applied either throughtransfer coating, or other operations including, for example, silkscreening. Because of its excellent release properties,polytetrafluoroethylene film may be employed as a temporary base forsilicone rubber coatings. Thus, silicone rubber may be applied to atransistor base through transfer coating technqiues utilizing apolytetrafluoroethylene film base pad.

The method of fabricating the mounting means of the present invention isalso advantageous from the standpoint of the preparation of elongatedstrips of material which may be broken-away into individual mounting padmeans. Furthermore, the utilization of silicone rubber adhesive padsfacilitates direct mounting of a solid-state semiconductive translatingdevice to the surface of the base pad. Such direct-mounting of asolid-state semiconductive translating device may be accomplished witheither strip-form assemblies or rolls on a reel-to-reel basis. Suchassembly techniques expedite production rates and provide forversatility in manufacture and assembly.

What is claimed is:
 1. Mounting means for use in combination withsolid-state semiconductive translating devices and comprising:(a) agenerally rigid deformable sheet element having at least one metal foillayer and including means defining a centrall disposed base pad havingedge surfaces including opposed ends and opposed sides, and flange wallscoupled to the opposed side edges and extending away from said base pad;(b) an adhesive menas secured to said sheet element and defining atleast one first mounting pad means secured to the surface of said basepad and being composed of a thin layer of silicone base rubber tofacilitate the direct mounting of a solid-state semiconductive device tosaid base pad; and (c) a second mounting pad means secured to said basepad on a side of the base pad opposite said first mounting pad means,for mounting said base pad with respect to a circuit board.
 2. Mountingmenas for use in combination with solid-state semiconductive translatingdevices and comprising:(a) a base pad having edge surfaces includingopposed ends and opposed sides, flange walls coupled to the opposed sideedges and extending away from said base pad; (b) said base pad andflange walls comprising a generally rigid laminate with a core havingouter metal foil layers disposed on opposite surfaces of the core, saidcore being composed of a thin layer of silicone base rubber; (c) a firstadhesive device mounting pad means being secured to the outer surface ofsaid base pad and being composed of a thin layer of silicone baserubber; and (d) a second mounting pad means secured to said base pad ona side of the base pad opposite said first mounting pad means, formounting said base pad with respect to a circuit board.
 3. An apparatusfor use in combination with a plurality of solid-state semiconductivetranslating devices and comprising:(a) an elongate metal layer includingat least one metal foil and a plurality of grooves formed transverselyacross said metal layer for dividing said layer into a plurality of basepads and a plurality of flange walls, with at least two of said flangewalls positioned between each pair of adjacent base pads, said metallayer being generally rigid and deformable; and (b) an adhesive meanssecured to said metal foil at said base pads and defining a first devicemounting pad means secured to each of said base pads and composed of athin layer of silicone base rubber, for the direct mounting of each of aplurality of solid-state semiconductor devices to one of said base pads.4. The mounting means of claim 3 wherein:said adhesive means furtherdefines a second mounting pad means secured to each of said base pads ona side of the associated base pad opposite an associated one of saidfirst mounting pad means, for mounting said base pads with respect to acircuit board.